VALIDATING THE DURABILITY OF CORROSION RESISTANT
MINERAL ADMIXTURE CONCRETE
Final Report
Prepared for the
by
Xianming Shi, Ph. D., P. E. ( Principal Investigator)
Yajun Liu, Ph. D.
Zhengxian Yang, M. Sc.
Michael Berry, Ph. D.
Prathish Kumar Rajaraman
Corrosion & Sustainable Infrastructure Laboratory
Western Transportation Institute
Montana State University, Bozeman, MT 59717
December 30, 2010
ii
DISCLAIMER
The contents of this report reflect the views of the authors, who are responsible for the
facts and the accuracy of the data presented herein. The contents do not necessarily
reflect the official views or policies of the California Department of Transportation
( Caltrans) or the Federal Highway Administration. This report does not constitute a
standard, specification, or regulation.
Reference herein to any specific commercial products, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the authors or the project sponsors.
Alternative accessible formats of this document will be provided upon request. Persons
with disabilities who need an alternative accessible format of this information, or who
require some other reasonable accommodation to participate, should contact Catherine
Heidkamp, Assistant Director for Communications and Information Systems, Western
Transportation Institute, Montana State University, PO Box 174250, Bozeman, MT
59717- 4250, telephone number 406- 994- 7018, e- mail: KateL@ coe. montana. edu.
.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support provided by the California Department of
Transportation as well as the Research & Innovative Technology Administration ( RITA)
at the U. S. Department of Transportation for this project. The authors are indebted to the
Caltrans Research Manager Peter S. Lee and the technical panel consisting of Rob Reis,
Doug Parks, Rudy Lopez, and Charlie Sparkman, for their continued support throughout
this project. We owe our thanks to Doran Glauz and Larry McCrum at Caltrans for
discussions related to the handling and preparation of the coarse and fine aggregates prior
to the batching operations. We appreciate the following professionals who provided
assistance to this research: Richard Sullivan ( Caltrans), Richard Halverson ( Headwaters
Resources), Steve Beck ( Western Pozzolan Co.), Jim Anderson ( BASF/ MB Admixtures),
Ken McPhalen ( Advanced Cement Technologies), Kevin Foody ( Boral Material
Technologies), Greg Juell ( Lehigh Southwest Cement Co.), and Jeff Wiest ( Ashgrove
Montana City Plant). We also thank Dr. Brett Gunnink of the MSU Civil Engineering
Department for coordinating the use of Bulk Materials Laboratory, Concrete Wet Curing
Room and other facilities. Finally, we owe our thanks to the following individuals at the
Western Transportation Institute for providing help in various stages of the laboratory
investigation: Marijean M. Peterson, Doug Cross, Dr. Tuan Anh Nguyen, Levi Ewan,
Andrea Beth Leonard, Matthew Mooney, and Eric Schon.
iii
1. REPORT NO. 2. GOVERNMENT ACCESSION NO. 3. RECIPIENTS CATALOG NO
CA-
4. TITLE AND SUBTITLE 5. REPORT DATE
Validating the Durability of Corrosion Resistant Mineral
Admixture Concrete
December 2010
6. PERFORMING ORGANIZATION CODE
7. AUTHOR( S) 8. PERFORMING ORGANIZATION REPORT NO.
Xianming Shi, Yajun Liu, Zhengxian Yang, Michael Berry, and
Prathish Kumar Rajaraman
9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. WORK UNIT NO.
Corrosion & Sustainable Infrastructure Laboratory
Western Transportation Institute
PO Box 174250, Montana State University
Bozeman, MT 59717- 4250
11. CONTRACT OR GRANT NO.
12. CO- SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED
California Department of Transportation
Research Manager: Peter S. Lee
Final Report, Jan. 2007- Dec. 2010
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
Conducted in cooperation with the U. S. Department of Transportation, Federal Highway Administration.
16. ABSTRACT
The objectives of this research are to validate chloride diffusion coefficients of mineral admixture concrete
mix designs currently developed by the Caltrans for corrosion mitigation, and to verify the adequacy of
existing measures to mitigate corrosion caused by exposure to marine environments and deicing salt
applications. To this end, this research includes a comprehensive literature review on relevant topics, a
laboratory investigation and a modeling effort. Various laboratory tests were conducted to investigate the
compressive strength of and chloride diffusivity in mortar and concrete samples with cement partially
replaced by various minerals ( class F and class N fly ash, ultra- fine fly ash, silica fume, metakaolin and
ground granulated blast- furnace slag), the porosity of mineral concretes, the freeze- thaw resistance of
mineral mortars in the presence of deicers, and the effect of mineral admixtures on the chloride binding
and chemistry of the pore solution in mortar. The modeling effort explores the important features of ionic
transport in concrete and develops a two- dimensional finite- element- method ( FEM) model coupled with
the stochastic technique. The numerical model is then used to examine the service life of reinforced
concrete as a function of mix design ( i. e., partial replacement of cement by mineral admixtures), concrete
cover depth, surface chloride concentrations, and presence of cracks and coarse aggregates.
17. KEY WORDS 18. DISTRIBUTION STATEMENT
Fly ash, silica fume, metakaolin, slag, mineral
admixtures, supplementary cementitious materials,
rebar corrosion, reinforced concrete, chloride ingress,
environmentally friendly concrete, service life
prediction
No restrictions. This document is available to
the public through the National Technical
Information Service, Springfield, VA 22161;
www. ntis. gov
19. SECURITY CLASSIF. ( of this report) 20. SECURITY CLASSIF. ( of this page) 21. NO. OF PAGES 22. PRICE
None None
iv
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v
Table of Contents
ACKNOWLEDGEMENTS .................................................................................................................. II
LIST OF FIGURES ........................................................................................................................... VII
LIST OF TABLES ............................................................................................................................ VIII
ABBREVIATIONS AND ACRONYMS ............................................................................................ IX
EXECUTIVE SUMMARY ........................................................................................................ 1
CHAPTER 1. INTRODUCTION ......................................................................................................... 7
1.1. PROBLEM STATEMENT ................................................................................... 7
1.2. BACKGROUND ................................................................................................. 9
1.2.1. Chloride- Induced Corrosion of Steel Rebar in Concrete .............................. 9
1.2.2. Role of Mineral Admixtures in Concrete Durability ................................... 10
1.2.3. Measuring the Chloride Ingress into Concrete ........................................... 16
1.3. CHALLENGES IN ASSESSING CONCRETE DURABILITY FROM ITS
CHLORIDE DIFFUSIVITY ............................................................................... 18
1.3.1. Chloride Threshold ...................................................................................... 18
1.3.2. Chloride Binding ......................................................................................... 20
1.4. CHLORIDE TRANSPORT IN CONCRETE AND SERVICE LIFE OF
REINFORCED CONCRETE – A MODELING PERSPECTIVE ............................ 21
1.5. A PHENOMENOLOGICAL MODEL FOR THE CHLORIDE THRESHOLD
OF PITTING CORROSION OF STEEL IN SIMULATED CONCRETE PORE
SOLUTIONS ................................................................................................... 21
1.6. MODELING CATHODIC PREVENTION FOR UNCONVENTIONAL
CONCRETE IN SALT- LADEN ENVIRONMENT ................................................ 21
1.7. STUDY OBJECTIVES ...................................................................................... 22
1.8. HOW THIS REPORT IS ORGANIZED ............................................................. 22
1.9. REFERENCES ................................................................................................ 23
CHAPTER 2. LABORATORY INVESTIGATION ......................................................................... 32
2.1. EXPERIMENTAL .......................................................................................................... 32
vi
2.1.1. Sample Preparation ........................................................................................ 32
2.1.2. Mechanical Testing ......................................................................................... 36
2.1.3. Electro- migration and Natural Diffusion ....................................................... 36
2.1.4. Electrochemical Impedance Spectroscopy ( EIS) Measurements .................... 40
2.1.5 Chloride Binding Capacity and Pore Solution Chemistry of Mortar Samples 41
2.1.6 Porosity Measurements of Concrete Samples .................................................. 41
2.1.7 Freeze- thaw Resistance of Mortar Samples .................................................... 41
2.2. RESULTS AND DISCUSSION ......................................................................................... 42
2.2.1 Mechanical Properties of Mortar and Concrete Samples and Correlation with
Chloride Diffusivity ................................................................................................... 43
2.2.2 Electro- migration Data and Correlation with Porosity .................................. 49
2.2.3. EIS Data of Concrete and Correlation with Chloride Diffusivity .................. 52
2.2.4. Chloride Binding of Mortar and Influence of Mineral Admixtures ................ 54
2.2.6. Freeze- thaw Resistance of Mortar in the Presence of Chlorides ................... 57
2.3. CONCLUSIONS ............................................................................................................. 58
2.4. REFERENCES ............................................................................................................... 59
CHAPTER 3. STOCHASTIC MODELING OF SERVICE LIFE OF REINFORCED
CONCRETE IN CHLORIDE- LADEN ENVIRONMENTS .................................. 60
3.1. INTRODUCTION ........................................................................................................... 60
3.2. METHODOLOGY .......................................................................................................... 61
3.2.1. Model for Multi- Species Transport ( Method A) ............................................. 62
3.2.2. Model for Single- Species Transport ( Method B) ............................................ 63
3.2.3. Input Parameters ............................................................................................ 64
3.3. RESULTS AND DISCUSSION ......................................................................................... 67
3.3.1. Effect of Mix Design on Service Life ............................................................... 67
3.3.2. Effect of Surface Chloride Concentration on Service Life .............................. 68
3.3.3. Effect of Cracking Level on Service Life ......................................................... 70
3.3.4. Effect of Coarse Aggregates on Service Life .................................................. 72
3.3.5. Effect of Concrete Cover Depth on Service Life ............................................. 74
3.4. CONCLUDING REMARKS ............................................................................................. 75
3.5. REFERENCES ............................................................................................................... 76
CHAPTER 4. CONCLUSIONS AND RECOMMENDATIONS FOR IMPLEMENTATION .... 79
4.1. CONCLUSIONS ............................................................................................... 79
4.2. RECOMMENDATIONS FOR IMPLEMENTATION ............................................. 83
vii
List of Figures
FIGURE 1- 1 A typical corrosion cell in a salt- contaminated reinforced concrete ............. 9
FIGURE 2- 1 Schematic illustration ( a) and photo ( b) of the experimental setup for
the electro- migration test ( Note: for the natural diffusion samples,
there is no external electric field applied). .................................................... 37
FIGURE 2- 2 Photos of the migration tests: ( a) electro- migration and ( b) natural
diffusion. ....................................................................................................... 38
FIGURE 2- 3 Temporal evolution of chloride concentration in the destination
compartment, with data obtained from the electro- migration test of 3%
NaCl through a Portland cement mortar specimen. ...................................... 39
FIGURE 2- 4 The equivalent circuit used for fitting the EIS data of mortar and
concrete ......................................................................................................... 40
FIGURE 2- 5 Relationship between compressive strength and chloride diffusion
coefficients: ( a) mortar samples and ( b) concrete samples. .......................... 47
FIGURE 2- 6 Relationship between: ( a) chloride diffusivity in mortar and that in
non- air- entrained concrete; ( b) transformed strength of mortar and 90-
day compressive strength of concrete. .......................................................... 48
FIGURE 2- 7 Temporal evolution of: ( a) chloride concentration in destination
compartment; and ( b) electric current density during the ACMT of
concrete samples. .......................................................................................... 50
FIGURE 2- 8 Correlation between: ( a) porosity and chloride diffusion coefficient
for concrete samples; ( b) cumulative electrical charge and chloride
diffusion coefficient. ..................................................................................... 51
FIGURE 2- 9 Correlation between the measured Qcement ( a) and Rcement ( b) against
chloride diffusion coefficients. ..................................................................... 53
FIGURE 2- 10 Chloride binding isotherms for four kinds of mortar samples. ................. 55
FIGURE 2- 11 Chloride binding percentage with respect to mix designs of different
mortar samples. ............................................................................................. 56
FIGURE 2- 12 Weight loss of concrete specimens after freeze- thaw tests in the
presence of various chlorides ........................................................................ 58
FIGURE 3- 13 Surface chloride concentration distribution according to the normal
distribution: ( a) 2 kg/ m3; ( b) 4 kg/ m3; ( c) 6 kg/ m3 and ( d) 8 kg/ m3.............. 65
FIGURE 3- 14 Chloride diffusion coefficient distribution according to the gamma
distribution. ................................................................................................... 66
FIGURE 3- 15 Chloride corrosion initiation concentration distribution according to
the triangular distribution. ............................................................................. 66
FIGURE 3- 16 Concrete cover depth distribution according to the normal
distribution. ................................................................................................... 66
FIGURE 3- 17 Predicted service life for concrete structures with surface chloride
concentrations being ( a) 2 kg/ m3, ( b) 4 kg/ m3, ( c) 6 kg/ m3 and ( d) 8
kg/ m3. ............................................................................................................ 69
FIGURE 3- 18 Crack configurations with various densities for service life
prediction: ( a) 50 m- 1; ( b) 80 m- 1; ( c) 145 m- 1 and ( d) 207 m- 1. .................... 71
FIGURE 3- 19 Effect of crack densities on service life prediction: ( a) 50 m- 1; ( b) 80
m- 1; ( c) 145 m- 1 and ( d) 207 m- 1.................................................................... 72
viii
FIGURE 3- 20 Effect of aggregate fraction on chloride- induced corrosion ( a) mix
design 2, ( b) mix design 4, ( c) mix design 8 and ( d) mix design 11. ............ 74
FIGURE 3- 21 Variation of service life with respect to concrete cover depth. ( a)
mix design 2, ( b) mix design 4 and ( c) mix design 8. ................................... 75
List of Tables
TABLE 1- 1 Summary of Chloride Penetration Test Methods [ 89] .................................. 16
TABLE 2- 2 Preliminary design of experiments to study the influence of concrete
mix design parameters on the chloride penetration resistance and
durability of concrete, including type and amount of mineral
replacement and entrained air content. ......................................................... 34
TABLE 2- 3 Mix design parameters and the properties of concrete samples
containing various types and amounts of mineral admixtures. ..................... 35
TABLE 2- 4 Mechanical properties, EIS data and chloride diffusivity of mortar
samples containing various types and amounts of mineral admixtures. ....... 39
TABLE 2- 5 The properties of mortar samples and concrete containing various
types and amounts of mineral admixtures. ................................................... 40
TABLE 2- 6 Chloride binding parameters measured from the mortar samples
containing various mineral admixtures. ........................................................ 56
TABLE 2- 7 Pore solution chemistry in mortar samples containing various types
and amounts of mineral admixtures. ............................................................. 57
TABLE 3- 8 Predicted service life for various mix designs, with a surface chloride
concentration of 6 kg/ m3 and concrete cover of 50 mm ............................... 68
ix
Abbreviations and Acronyms
AASHTO American Association of State Highway and Transportation Officials
ACMT accelerated chloride migration test
AgCl silver chloride
ANN artificial neural network
ASR alkali- silica reaction
ASTM American Society of Testing and Materials
BDS Bridge Design Specifications
BP back- propagation
 C degrees Celsius
C3A tricalcium aluminate
C4AF Friedel’s salt, 3CaO  Al2O3  CaCl2  10H2O
CaCl2 calcium chloride
Caltrans California Department of Transportation
Ca( OH) 2 calcium hydroxide
Clth chloride threshold
CP cathodic protection
CPre cathodic prevention
C- S- H calcium silicate hydrate
Dapp apparent diffusion coefficient
Deff effective diffusion coefficient
Dns non- steady- state diffusion coefficient
Ds steady- state diffusion coefficient
DO dissolved oxygen
DOT Department of Transportation
Ecorr corrosion potential
EDTA ethylenediaminetetraacetic acid
EDX energy dispersive x- ray spectroscopy
EFCs environmentally friendly concretes
EIS electrochemical impedance spectroscopy
FA fly ash
FDM Finite Difference Method
FEM Finite Element Method
 F degrees Fahrenheit
FESEM field emission scanning electron microscopy
FHWA Federal Highway Administration
GGBFS ground granulated blast- furnace slag
HCl hydrochloric acid
x
IC/ ICP Chromatography- Inductively Coupled Plasma
ITZ interfacial transition zone
LOI loss on ignition
MgCl2 magnesium chloride
MK metakaolin
NACE National Association of Corrosion Engineers
NaCl sodium chloride
NCHRP National Center for Highway Research Program
OCP open circuit potential
OH- hydroxyl
PCC Portland cement concrete
PDEs partial differential equations
RCPT rapid chloride permeability test
RMT rapid migration test
SCC self- compacting concrete
SCE saturated calomel electrode
SCMs supplementary cementitious materials
SF silica fume
SHRP Strategic Highway Research Program
SMSE sum of mean square error
SSD surface- saturated- dry
Ti time- to- corrosion ( initiation time)
TMS transformed mortar strength
UFFA ultra- fine fly ash
w/ c water- to- cement ratio
w/ cm water- to- cementitious- materials ratio
WTI Western Transportation Institute
1
EXECUTIVE SUMMARY
Prior to this work, Caltrans saw the need for research to validate the corrosion mitigation
design assumptions in order to better define the strategies used to design concrete
structures with adequate corrosion mitigation measures and thus a “ maintenance- free”
service life. Additional research was also considered necessary to establish standard,
reliable, and rapid test methods for determining chloride diffusion coefficients and
chloride thresholds.
In this work, various laboratory tests were conducted to investigate the properties of
mortar and concrete samples with cement partially replaced by various minerals ( class F
and class N fly ash [ FA], ultra- fine fly ash [ UFFA], silica fume [ SF], metakaolin [ MK]
and ground granulated blast- furnace slag [ GGBFS]). The key findings are provided as
follows. These include: the compressive strength, Young's modulus, and modulus of
toughness of mortar samples at 1- d, 7- d and 28- d; the compressive strength and porosity
of concrete samples at 90- d; the chloride diffusivity and EIS measurements of hardened
mortar and concrete samples; the natural diffusion of chloride into select concrete
samples; the freeze- thaw resistance of mortars in the presence of chloride deicers; and the
effect of mineral admixtures on the chloride binding and chemistry of the pore solution in
mortar.
The accelerated chloride migration test of hardened concrete specimens found them to
feature unusually low chloride diffusivity ( Ds values in the order of 10- 13 m2/ s vs. the
commonly reported 10- 12 m2/ s), corresponding to very high compressive strength. The
research findings imply that these high- quality concrete samples tested likely had little or
no interfacial transitional zone ( ITZ) in them. The chloride diffusivity in high- strength
concretes was largely determined by the use of coarse aggregates whereas the effect of
mineral admixtures was relatively small.
Some detailed findings from the laboratory investigation are provided as follows.
1. The partial replacement of cement by 20% class F FA and 5% SF, by 20% class F
FA and 5% MK, or by 25% class F FA alone greatly reduced the 1- day
compressive strength of mortar samples, whereas the partial replacement of
cement by 10% MK, 10% SF, 10% UFFA, 50% GGBFS, or 25% class N FA
improved the 1- day strength to various degrees.
2. The combined addition of class F and MK dramatically reduced the 7- day
compressive strength of mortar samples, followed by the use of GGBFS or SF,
whereas the addition of most other minerals ( except MK) also decreased the 7- day
strength to various degrees.
3. The combined addition of class F and MK increased the 28- day compressive
strength of mortar samples, whereas the addition of most other minerals ( except
GGBFS) decreased the 28- day strength to various degrees.
4. All the SCMs dramatically reduced the 1- day Young's modulus of mortar
samples, but they showed mixed effect on the 7- day and 28- day Young's
2
modulus. All the SCMs dramatically reduced the 7- day and 28- day modulus of
toughness, but they showed mixed effect on the 1- day modulus of toughness.
5. According to the EIS measurements after the ACMT using 90- day old mortar
samples, all the SCMs dramatically increased the electrical resistivity of the
mortar samples in the electrolyte while most SCMs ( except GGBFS) decreased
the electrical capacitance of the mortar to various degrees.
6. The effect of partially replacing cement with SCMs on the steady- state diffusion
coefficient ( Ds) obtained from the ACMT was evaluated using 90- day old mortar
samples. The results indicate that the use of 20% class F FA and 5% SF as cement
replacement significantly increased the chloride diffusivity in mortar and the use
of 10% MK or 50% GGBFS significantly decreased it, whereas other SCMs
decreased the Ds to various degrees. Note that the Ds values were all very low ( in
the order of 10- 13 m2/ s), and the chloride diffusivity differences between these
highly impermeable concrete samples could be related to the workability and
construction practices of the fresh concrete mixes.
7. There is no clear trend related to the effect of SCMs on the 90- day compressive
strength of concrete or the chloride diffusivity in the 360- day concrete samples.
Nonetheless, the chloride diffusivity is much lower in the concrete mixes than in
their corresponding mortar mixes, with the Ds values in the order of 10- 13 m2/ s in
concrete and of 10- 11 m2/ s in mortar. This highlights the important role of coarse
aggregates in slowing down the chloride ingress into concrete.
8. All the mortar mixes had a 28- day compressive strength above 4,000 psi ( 27.6
MPa) whereas the non- air- entrained concrete mixes at 90 days on average
featured twice as high a compressive strength. Such extremely high strength
values suggest that the hardened concrete had outstanding microstructure, which
is consistent with their extremely low Ds values indicative of chloride diffusivity.
The compressive strength of air- entrained concrete was consistently lower than
that of their non- air- entrained counterpart, yet the differences in their chloride
diffusivity were not as appreciable.
9. The natural diffusion results indirectly confirmed the order of magnitude of Ds
values of concrete specimens obtained from the ACMT.
10. Generally speaking, the lower Ds values corresponded to the higher compressive
strength values, as both indicate high quality of the mortar or concrete. The lower
Ds values in mortar corresponded to the lower Ds values in the non- air- entrained
concrete, indicating that chloride diffusion in the mortar phase contributed to the
overall chloride diffusion in the concrete.
11. There is a strong proportional correlation between the transformed mortar strength
and the concrete strength, suggesting that the mortar phase is an integral
component of the heterogeneous concrete matrix and greatly contributes to its
compressive strength.
12. The chloride diffusivity generally increases with the volume of permeable voids
in concrete.
13. The cumulative charge generally increases with chloride diffusion coefficients.
3
14. The electrical resistivity of concrete generally decreased after the electro-migration
test whereas its electrical capacitance generally increased.
15. Mix design 9 ( 20% class F FA + 5% MK) and mix design 11 ( 10% SF) had the
lowest binding capacity, whereas mix designs 1 ( 100% cement), 7 ( 10% UFFA)
and 9 ( 50% GGBFS) had generally high chloride binding capacity relative to
other mixes.
16. The pH data suggest that all the mineral admixtures reduced the alkalinity of the
pore solution in the mortar samples, regardless of their type and amount.
17. The weight loss of mortar specimens was the greatest in the presence of diluted
NaCl solution, followed by the diluted CaCl2, and then by the diluted MgCl2
solution, whereas the mortar deterioration in the de- ionized water was negligible.
In the presence of diluted NaCl solution, the mix designs, the mix designs 13
( 10% MK), 15 ( 10% UFFA) and particularly 17 ( 50% GGBFS) showed less
weight loss relative to the control, whereas other SCMs exacerbated the freeze-thaw
damage with the mix designs 5 ( 25% class N FA) and particularly 7 ( 20%
class F FA + 5% SF) being the worst. In the presence of diluted CaCl2 and MgCl2
solutions, the effect of mineral admixtures on the freeze- thaw resistance of mortar
followed a trend similar to that seen in the presence of diluted NaCl solution, yet
with the mix designs 5 ( 25% class N FA) and 7 ( 20% class F FA + 5% SF) being
the worst respectively. In summary, the partial replacement of cement by 50%
GGBFS is most beneficial for the freeze- thaw resistance of mortar, followed by
the 10% UFFA or 10% MK replacement; whereas the use of ordinary fly ash and
silica fume seems to undermine the freeze- thaw resistance of mortar in the
presence of various diluted chloride solutions.
A two- dimensional finite- element- method ( FEM) model, coupled with the stochastic
technique, was developed to study the service life of reinforced concrete as a function of
various influential factors. The FEM model stochastically sampled its inputs.
Specifically, the surface chloride concentrations and concrete cover depth follow the
normal distribution, whereas the diffusion coefficients and the chloride threshold follow
the gamma distribution and the triangular distribution respectively. The nonlinear partial
differential equations ( PDEs) to characterize the spatial and temporal evolution of ionic
species were numerically solved. The key findings are provided as follows.
1. All concrete mixes investigated had a 50%- probability service life of 114 years or
longer ( with a surface chloride concentration of 6 kg/ m3 and concrete cover of 50
mm), which highlights the great potential of reinforced concrete as a construction
material when the concrete is made using the best practices of construction and
curing and is free of cracking. The modeling also suggest that when the concrete
is made using the best practices, partially replacing cement with class F FA, SF,
MK, or GGBFS tends to decrease the service life of reinforced concrete or at least
show little benefits to its service life. This trend contradicts what have been
generally reported in published literature, and is likely attributable to the fact that
the Portland cement concrete ( PCC) made without any mineral admixtures ( mixes
1 and 2) featured unusually low chloride diffusion coefficients in the order of 10-
4
13 m2/ s. Finally, for all these high- quality concrete mixes, the effects of air
entrainment on the chloride diffusivity in concrete and the service life of
reinforced concrete were not dramatic and do not show a clear trend.
2. Based on the modeling results, chloride inward- diffusion with the lowest surface
chloride concentration is the most sluggish process. With the surface chloride
concentration increasing, the service life decreases significantly.
3. The service life of reinforced concrete decreases as the cracking level of the
concrete increases. When the crack density is over 200 m- 2, the service life shows
no significant dependence on further increase on crack density, which is
attributable to the forming of continuous net- like configuration in the concrete
domain.
4. Assuming negligible diffusion of chloride ions in coarse aggregates and absence
of interfacial transition zone ( ITZ), the chloride diffusion rate in concrete was
found to be quite different from its corresponding homogeneous medium. The
overall flux decreases as the volume fraction of aggregates increases.
5. For all the mineral concrete mixes investigated, as the concrete cover depth
increases, the time to corrosion initiation of rebar in concrete is predicted to
increase exponentially, highlighting the importance of cover depth in extending
the service life of reinforced concrete exposed to external chlorides. According to
the model calculations, it would take more than 100 years for the chloride ions in
an aggressive environment ( with surface chloride content of 8 kg/ m3) to reach the
threshold level at a depth of 60 mm. It should be cautioned that the chloride
diffusivity data used for the model were measured using specimens cored from
the center of a large concrete sample. In field construction, the top layer of the
concrete cover is likely to have much higher chloride diffusivity than the interior
of the concrete, in light of the possible defects derived from bleeding and water
evaporation etc. at the top layer. Furthermore, the field construction is unlikely to
achieve the same level of detailed quality assurance as implemented in the
laboratory study and cracking cannot be fully eliminated for the service life
prediction considerations. In this context, a thicker- than- predicted concrete cover
is needed for the target service life of concrete structures in the field environment
and the importance of good construction and curing practices can not be
overemphasized.
6. The technique developed in this work ( e. g., multi- species transport model) was
found to be very effective in predicting chloride migration and generating
statistical conclusions about the service life of reinforced concrete, which allows
the civil engineers to estimate the rate of chloride ingress and associated
deterioration risk of reinforced concrete. Future improvements could be made to
the model so that it takes into account the time- dependency of transport properties
of concrete, the corrosion propagation, the chloride penetration mechanisms other
than diffusion ( e. g., wicking), the structure geometry, the environmental humidity
and temperature fluctuations and the decay of structures under coupled physical,
chemical, and mechanical deterioration processes etc.
5
Recommendations for Implementation
1. The accelerated chloride migration test ( ACMT) used in this work should be
considered by the Caltrans corrosion technology branch for implementation.
When testing the concrete with unusually low chloride diffusivity ( Ds values in
the order of 10- 13 m2/ s), the test could last up to 2 months using a 30- V applied
voltage and a 25- mm thick disc specimen. Nonetheless, for most concrete mixes
prepared in the field construction, the chloride diffusivity is expected to be much
higher and the test typically would last no more than 2 weeks. An unusually high
compressive strength can serve as a warning sign that the concrete may be highly
impermeable. In general, the ACMT is anticipated to help Caltrans and other
departments of transportation ( DOTs) to make the transition from prescriptive
specifications of concrete mixes to more performance- based specifications, which
then would allow more innovation and flexibility in the materials selection of
concrete and likely facilitate the paradigm shift from conventional PCC to EFCs.
2. If coupled the ACMT with the model developed in this work or the simplistic
Life- 365 software, this would provide a rapid, reliable method for determining the
amount of concrete cover needed, based on the amount of chlorides present in the
service environment and the required design life. With further improvements on
the service life model, it could also be used for life cycle costing and for the
timing of repair or rehabilitation strategies.
3. Caltrans should consider additional research phases for this work, such as the
development and field evaluation of various types of high performance corrosion-resistant
concretes. The research findings from such work should be shared with
the DOT Design Engineers, as it may lead to improvements to the current Bridge
Design Specifications in mitigating chloride- induced corrosion and deterioration.
4. The research findings imply that the chloride diffusivity in high- strength
concretes largely determined by the use of coarse aggregates instead of the
mineral admixtures. As such, the role of coarse aggregates in concrete durability
should be further explored. The existing ASTM standard on the proportioning of
aggregates may be further optimized for conventional and unconventional
concrete mixes, in light of their important role in dramatically slowing down
chloride ingress. Similarly, how the preparation of aggregates affects the
durability of concrete merit further investigation, as it may benefit the internal
curing of concrete and minimize its early- age cracking.
5. The processes and procedures used in the new construction should be closely
supervised under a systematic quality assurance program, in order to achieve the
great potential of reinforced concrete as a construction material and to manage
corrosion risks pro- actively. The importance of good construction and curing
practices can not be overemphasized, as they greatly reduce the risk of rebar
corrosion in concrete.
6. Continued research is needed to explore the effect of partially replacing cement
with mineral admixtures on the durability of concrete. The results from this study
imply that for concrete with ordinary quality, the mineral admixtures may have
6
great potential in increasing its electric resistivity, enhancing its chloride binding
( e. g., the use of 10% UFFA or 50% GGBFS), reducing its chloride diffusivity
( e. g., the use of 10% MK or 50% GGBFS), and improving its resistance to freeze-thaw
in the presence of diluted deicers ( e. g., the use of 50% GGBFS, 10% UFFA,
or 10% MK). The use of fly ash and slag etc. may translate to cost savings and
reduced energy use, greenhouse gas emissions and landfill waste, without
sacrificing quality and long- term performance of the concrete.
7
CHAPTER 1. INTRODUCTION
1.1. Problem Statement
Concrete is the most widely used man- made building material in the world, owing to its
versatility and relatively low cost. Concrete has also become the material of choice for
the construction of structures exposed to extreme conditions [ 1]. Furthermore,
sustainability has become an increasingly important characteristic for concrete
infrastructure, as the production of Portland cement ( the most common binder in
concrete) is an energy- intensive process that accounts for a significant portion of global
carbon dioxide emissions and other greenhouse gases [ 2,3]. As such, even slight
improvements in the design, production, construction, maintenance, and materials
performance of concrete can have enormous social, economic and environmental
impacts.
In this context, there are a variety of approaches to enhance the sustainability of concrete
and reduce its environmental footprint. One attractive approach is to use unconventional
binder such as fly ash and other industrial byproducts as a replacement for the Portland
cement in concrete. Another approach is to enhance the durability of concrete
infrastructure, since durability is a key cornerstone for sustainability. According to the
ASCE 2009 Report Card for America’s Infrastructure, $ 2.2 trillion needs to be invested
over five years to ‘ bring the nation’s infrastructure to a good condition” [ 4], which
highlights the urgent need for research devoted to longer- lasting and “ maintenance- free”
concrete materials.
There is general agreement that the most effective improvement in concrete durability
can be achieved at the design and materials selection stage of a project by using adequate
concrete cover and high- quality concrete. Usually, an increase in the thickness of the
concrete cover leads to beneficial effects, because it increases the barrier to the various
aggressive species moving towards the reinforcement and increases the time for corrosion
to initiate. In reality, however, the cover thickness cannot exceed certain limits, for
mechanical and practical reasons [ 5]. The Florida Department of Transportation ( DOT)
adheres to the following specifications for concrete bridge substructures within the 0- 12
foot elevation range relative to mean high tide: 1) adequate cover ( 4 inches for cast- in-place
members and 3 inches for prestressed components), and 2) low water- to-cementitious
material ( w/ c) ratio concrete with pozzolanic ( fly ash or silica fume) or
corrosion inhibiting admixtures [ 12]. In light of advances in concrete technology and
requirements of the AASHTO Load and Resistance Factor Design ( LRFD) for a 75- year
design life, the California Department of Transportation ( Caltrans) made significant
changes to its Bridge Design Specifications ( BDS) Article 8.22 in 2000 and adopted the
approach of using the chloride diffusivity through concrete to determine the concrete
cover requirements for structures subjected to chloride- bearing environments. The
current BDS Article 8.22 provides guidance to the Design Engineer in determining the
required cement type, minimum required concrete cover, etc. for corrosion protection of
various bridge members [ 6]. For instance, for bridge members exposed to corrosive soil
or water ( containing more than 500 ppm of chlorides), the maximum w/ c ratio shall not
8
exceed 0.40. Mineral admixtures conforming to ASTM Designation C 618 Type F or N
( e. g., fly ash) are required for all exposure conditions, except for ‘ non- corrosive’
exposure conditions. For such bridge members as precast piles and pile extensions
exposed to corrosive conditions, mineral admixtures conforming to ASTM Designation C
1240 ( e. g., silica fume) may be required. The minimum concrete cover required for
bridge members ranges from 1 to 5 inches, dependent on the bridge member type and
exposure condition [ 6].
Recent years have seen increasing interest in environmentally- friendly concretes ( EFCs),
which utilize industrial byproducts or waste materials and thus benefit the environment.
Among them, mineral admixtures such as fly ash, silica fume, and slag – have been used
to partially replace cement in concrete while shown to enhance concrete durability and
improve resistance to chloride diffusion. They are also known as supplementary
cementitious materials, or SCMs. Like other state DOTs, Caltrans has developed concrete
mixes for corrosion mitigation of structures with the aid of such SCMs. However, the
work to date has been based on diffusion coefficient data for low permeability, mineral
admixture concretes selected from available literature, which may not represent the
materials and exposure conditions seen in California. Additional research is thus needed
to validate the corrosion mitigation design assumptions by Caltrans in order to better
define the strategies used to design concrete structures with adequate corrosion mitigation
measures and thus a “ maintenance- free” service life.
Furthermore, a significant amount of variability exists in determining chloride diffusion
coefficients as an indicator of concrete durability. First, values of chloride diffusion
coefficient usually vary from 10- 13 m2/ s to 10- 10m2/ s in relation to the concrete properties
and the exposure conditions. In particular, these values depend on the concrete pore
structure and on all the factors that determine it, such as: mix design parameters ( w/ c
ratio, type and proportion of mineral admixtures and cement, compaction, curing, etc.)
and presence of cracks. The chloride diffusion coefficient is also a function of chloride
exposure condition ( submerged, splash, atmosphere, etc.) and the length of exposure,
partly due to hydration of slowly reacting cement constituents such as blast furnace slag
or fly ash [ 5]. When the chloride diffusion coefficient is used to evaluate the risk for
reinforcement corrosion and to forecast the service life of concrete structures, chloride
threshold is a very important parameter, the value of which is still a subject of
controversy. In reality, the determination of chloride diffusion coefficient and chloride
threshold is often affected by the method of chloride analysis. Second, existing chloride
permeability tests are either very time- consuming for high- quality concrete mixes or too
biased to provide reliable chloride diffusion coefficients. Additional research is thus
needed to establish standard, reliable, and rapid test methods for determining chloride
diffusion coefficients and chloride thresholds. Such methods are anticipated to help
Caltrans and other DOTs to make the transition from prescriptive specifications of
concrete mixes to more performance- based specifications, which then would allow more
innovation and flexibility in the materials selection of concrete and likely facilitate the
paradigm shift from conventional Portland cement concrete ( PCC) to EFCs.
9
1.2. Background
1.2.1. Chloride- Induced Corrosion of Steel Rebar in Concrete
Concrete normally provides both chemical and physical protection for the steel
reinforcement embedded in concrete. The cement hydration leads to the highly alkaline
( pH 13 or higher) pore solution of concrete, which promotes the formation of an
oxide/ hydroxide film at the steel surface, a passive film of about 10 nanometers thick [ 7].
This protective film effectively insulates the steel and electrolytes so that the corrosion
rate is negligible. In addition, the concrete cover prevents or at least retards the ingress of
aggressive substances. While the chloride ion ( Cl-) has only a small influence on pore
water pH, concentrations as low as 0.6 kilograms per cubic meter ( kg/ m3 by weight of
concrete) have been projected to compromise steel passivity [ 7]. Furthermore, the
protection of steel by concrete is compromised by the gradual ingress of atmospheric
carbon dioxide into the concrete, a process known as carbonation, which reduces the
pore solution pH of carbonated concrete to the range of 8 to 9. The corrosion due to
carbonation progresses at a much lower rate that that due to chloride ingress [ 8]. Concrete
exposure to acids, sulfates and freeze- thaw cycles may also compromise the protection of
steel rebar.
Chloride ingress is one of the major forms of environmental attack to reinforced concrete
[ 9], which leads to corrosion of the reinforcing steel and a subsequent reduction in the
strength, serviceability, and aesthetics of the structure. For reinforced concrete structures
such as highway bridges, the chloride- induced corrosion of rebar has been a major
problem with serious economic and safety implications. Chloride, often originated from
marine environments or deicing applications, can initiate rebar corrosion once its
concentration at the embedded rebar depth reaches a certain threshold. The local
disruption of the passive film initiates corrosion cells between the active corrosion zones
( anode) and the surrounding areas that are still passive ( cathode) [ 7,10], as shown in
Figure 1- 1.
FIGURE 1- 1 A typical corrosion cell in a salt- contaminated reinforced concrete
10
For stable pit growth to be sustained, the relative concentrations of aggressive Cl- and
inhibitive OH- should be above a certain ratio, otherwise re- passivation will occur [ 11].
The accumulation of corrosion products ( oxides/ hydroxides) in the concrete pore space
near the rebar then builds up hoop stresses around steel and results in cracking or spalling
of the concrete, which in turn facilitates the ingress of moisture, oxygen, and chlorides to
the embedded rebar and accelerates the corrosion of steel [ 12].
Corrosive agents, liquid or gaseous, may penetrate the concrete through capillary
absorption, hydrostatic pressure, or diffusion. The ingress of gases, water or ions in
aqueous solutions into concrete takes place through pore spaces in the cement paste
matrix and paste- aggregate interfaces or microcracks. For the durability of concrete,
permeability is believed to be the most important characteristic [ 13], which is related to
its microstructural properties, such as the size, distribution, and interconnection of pores
and microcracks [ 14]. For reinforced concrete structures exposed to salt- laden
environments, the chloride permeability of concrete has been recognized as a critical
intrinsic property of the concrete [ 15].
1.2.2. Role of Mineral Admixtures in Concrete Durability
Mineral admixtures, generally pozzolanic materials, are mainly glassy siliceous materials
that may contain aluminous compounds [ 5]. The reaction of such materials with
Portlandite ( i. e., calcium hydroxide) and water generates hydration products similar to
those of Portland cement, i. e., calcium silicate hydrates ( C- S- H), a rigid gel composed of
extremely small particles with a layer structure:
pozzolan + water + Ca( OH) 2  C- S- H
This reaction can also be generally represented as:
H4SiO4 + Ca( OH) 2—> H2SiO4
2- + Ca2+ + 2 H2O —> CaH2SiO4 · 2 H2O
whereas the actual stoichiometry of the reaction may vary as a function of the Ca/ Si ratio,
available water molecules, etc., resulting in various C- S- H that may deviate from the
general formula ( CaH2SiO4 · 2 H2O).
The use of mineral admixtures such as fly ash, silica fume, slag and metakaolin has been
shown to enhance concrete durability [ 16- 18], by increasing chloride binding [ 19],
decreasing chloride permeability [ 18, 20], elevating threshold chloride content [ 21],
and/ or improving the distribution of pore size and shape of concrete matrix [ 22]. Since
some of these materials are cheaper than Portland cement, there is also an economic
advantage to wider use. Dhir and Jones [ 24] used the low- lime fly ash to develop
concrete mixes with improved chloride resistance, by improving the pore structure and
binding capacity of the concrete. They found that ternary blends ( cement and fly ash
blended with silica fume or metakaolin) showed the highest chloride resistance. Hossain
et al. [ 25] found that the incorporation of ultra- fine fly ash ( UFFA) improved the strength
and chloride penetration resistance of concrete, and the incorporation of silica fume had
11
even more pronounced benefits. They also found that the silica fume addition led to low
slump and high early- age shrinkage whereas the UFFA addition mitigated these two
issues. As such, a ternary blend ( with cement, silica fume and UFFA) was developed to
feature high early- age strength, improved durability, low slump and low free shrinkage.
Thomas et al. [ 26] investigated the synergy between silica fume and fly ash, as silica
fume compensates for the low early- strength pertinent to fly ash addition while fly ash
compensates for the workability issues pertinent to silica fume addition. The
combinations of low dosage ( 3% to 6%) of silica fume and moderate dosage ( 20% to
30%) of fly ash ( despite its high lime content) were found very effective in reducing
expansion due to alkali- silica reaction ( ASR) and in mitigating sulfate attack. The ternary
blends showed significant improvements in reducing chloride penetration and such
reduction in diffusivity continued to increase with age. Thomas and Bamforth [ 20]
modeled the chloride diffusion in concrete using data from long- term field and laboratory
studies and showed that the incorporation of fly ash and slag may have little influence on
the early- age chloride resistance but dramatic benefits after a few years of exposure.
Mangat et al. [ 27] investigated the partial replacement of cement by pulverised fuel ash,
slag, and microsilica and showed microsilica to be the most effective in enhancing the
corrosion resistance of rebar in concrete. The microsilica addition was found to greatly
increase the pore volume in cement paste yet greatly decrease the chloride penetration.
Güneyisi et al. [ 28] investigated the rebar corrosion in concrete made of blended cements
which contained various proportions of Portland cement clinker, blast furnace slag,
natural pozzolans, and limestone powder. Relative to the plain cement concrete, the
specimens with blended cements showed superior corrosion performance and generally
longer time to corrosion cracking, which correlated very well with the splitting tensile
strength data.
There is existing research demonstrating the use of mineral admixtures to improve other
aspects of concrete durability, such as effectively mitigating the ASR- induced damage in
concrete [ 29- 32]. Papadakis [ 33] found that replacing aggregates with SCMs ( silica fume,
low- and high- lime fly ashes) improves the resistance of concrete to carbonation whereas
replacing cement with SCMs increases the carbonation depth. In both cases, however, the
incorporation of SCMs significantly lowered the total chloride content in concrete at all
depths other than the very external surface layer. Mineral admixtures may slow the rate
of strength gain in concrete, but do not adversely affect the long- term concrete strength
[ 34] or even improve its strength properties [ 27, 35, 36]. Concrete mixes with high-volume
fly ash or high- volume fly ash and ground slag demonstrate good workability,
high compressive strength, and excellent durability ( negligible carbonation and very low
chloride penetration) [ 37].
Fly ash ( FA) is a byproduct of coal combustion in the generation of electricity, i. e., a
finely segregated residue captured from the flue gas at coal- fired power plants. Most FA
particles are spherical and amorphous, ranging in size between 10 and 100 microns. With
increasing energy costs and heightened concerns about the impact of concrete
construction and maintenance activities on the environment, there has been an attendant
increase in interest and research activity on the use of FA and other recycled materials in
concrete, including those targeting ASR prevention [ 38- 40]. The effectiveness of FA in
12
mitigating mortar bar expansion induced by potassium- acetate- based deicer solution was
found to depend on the lime content of FA and its dosage level [ 41].
The use of FA as a supplemental binder in concrete is common: 15 of the 72 million tons
of fly ash produced in the U. S. in 2006 were used for this purpose [ 42] and many states
have allowed the use of performance- specified ( ASTM C 1157) cements that contain FA.
The efficacy of a particular FA in this regard however is difficult to predict and no single
index value or combination of values is an infallible predictor of its performance in
concrete [ 43]. Following the provisions of ASTM C 618, fly ashes can be divided into
two primary classes, F and C, based on their chemical composition resulting from the
type of the coal burned. Normally Class F FA is produced from anthracite or bituminous
coals, whereas Class C FA is produced from lignite or sub- bituminous coals [ 44]. ASTM
C 618 also specifies another class, N, typically for natural pozzolans. This classification
system, based on the silica, alumina, and iron oxide content of the ash ( as shown in
Appendix E1), only indirectly indicates how the ash will behave as an ingredient in
concrete. Additional characteristics of importance include the calcium oxide content,
fineness, crystalline structure, and loss on ignition ( LOI, an indicator of carbon content)
of the ash.
There are numerous studies on the effect of FA addition on the durability of concrete.
Hedegaard and Hansen [ 45] argued that replacing cement with FA is likely degrade the
water- tightness of concrete, as they found that 1 kg of cement would have to be replaced
by 3 kg of FA in order to maintain the same level of water permeability of hardened
concrete ( at 28 days and 56 days). Wong et al. [ 46] tested notched mortar specimens and
concluded that a 15% cement replacement by class F FA enhanced the bond strength of
mortar- aggregate interface and fracture toughness. At high replacement levels ( 45% and
55%) the FA addition reduced the interfacial bond strength and fracture toughness at 28
days but such reductions were recovered at 90 days. The FA replacement at all levels was
found to increase the interfacial fracture energy. Gebler and Klieger [ 47] found that the
use of certain fly ashes degraded the freeze- thaw resistance of air- entrained concrete
when cured at low temperature and showed no significant influence for other conditions.
The incorporation of FA in concrete generally reduced the resistance of air- entrained
concrete to deicing scaling and showed little benefit on its resistance to chloride
penetration.
In general, FA addition in concrete is considered an effective measure to mitigate
chloride- induced corrosion of rebar in concrete. For instance, using FA blended cement is
known to reduce chloride permeability and improve sulfate resistance of concrete [ 48].
Dhir et al. [ 49] used the equilibrium method and found that the chloride binding capacity
of cement paste increased with the increase in FA replacement level up to 50% and then
declined at 67%. In the case of admixed chloride, the increase in chloride binding due to
the replacement of FA was also found [ 50- 53]. An increase in chloride binding may be
mainly ascribed to the high alumina content in FA [ 24, 51], which results in the
formation of more Friedel’s salt [ 54]. The increase in chloride binding could also be
ascribed to the production of more gel during hydration, which results in better physical
adsorption of chloride [ 55]. Other researchers [ 50, 51, 54] also found that partial
13
replacement of cement with FA has a positive effect on the chloride binding when the
cement paste was exposed to a chloride environment. However, Nagataki et al. [ 56]
found that the 30% replacement of cement with FA reduced the chloride binding capacity
of cementitious material in the case of external chlorides. Ampadu et al. [ 57] found the
partial replacement of cement by FA only showed significant benefits in reducing the
chloride diffusivity in cement paste at later ages of curing and a 40% replacement level
was the best. Thomas [ 58] reported that chloride threshold decreased with increasing of
FA content in marine exposure. These threshold values obtained were 0.7%, 0.65%, 0.5%
and 0.2% acid soluble chloride ( by mass of cementitious material) for concrete with 0,
15%, 30% and 50% FA, respectively. Despite of these lower threshold values, FA
concrete was found to provide better corrosion protection to steel rebar due to of its
higher resistance to chloride penetration. Oh et al. [ 59] also reported lower chloride
threshold values with increasing addition of FA, whereas Schiessl and Breit [ 60] and
Alonso et al. [ 61] reported higher or similar threshold values respectively when replacing
cement with FA. For a concrete mix with water- to- cementitious- materials ( w/ cm) ratio of
0.37, the addition of FA ( 35% cement replacement) and silica fume ( 27% cement
replacement) reduced the chloride diffusion coefficient from 3.48  10- 12 m2/ s to 7.35  10-
13 m2/ s and 1.01  10- 12 m2/ s, but also reduced the pore water pH from 13.84 to 13.39 and
13.47, respectively [ 12]. Other researchers [ 62, 63] also reported the reduction of pH in
the pore solution as a result of FA addition. The reduction of pore water pH may explain
the research finding that the chloride threshold decreased with increasing FA content in
concrete, whereas the improved resistance to chloride diffusion may explain the enhanced
protection of embedded steel by the FA admixed concrete [ 23]. Saraswathy and Wong
[ 64] investigated the effect of admixing activated FA on the corrosion resistance of
concrete and found that the FA addition significantly improved the corrosion
performance of concrete up to a critical moderate replacement level ( 20% to 30%) and
the chemical activation of FA worked the best.
Ultra- fine fly ash ( UFFA) is a relatively new pozzolanic admixture and there are a limited
number of studies on the effect of its addition on the durability of concrete. It is
processed from ordinary FA to obtain finer particles ( as shown in Appendix E5). UFFA
has been shown to feature higher pozzolanic activity than ordinary fly ashes, to greatly
reduce the water demand and air content of concrete, and to produce concrete of higher
strength and lower porosity [ 65, 66]. Hossain et al. [ 67] found that the restrained mortars
containing UFFA or ordinary class F fly ash had lower residual stress levels, less free
shrinkage, increased cracking age, and decreased creep effect, relative to the control. The
UFFA addition led to more pronounced delay in the age of cracking and in the reduction
in creep effect, relative to the ordinary fly ash. Subramaniam et al. [ 68] observed “ a
significant reduction in the autogenous shrinkage and an increase in the age of restrained
shrinkage cracking” in the concrete admixed with UFFA, relative to the control and the
concrete admixed with silica fume. An “ increase in the age of restrained shrinkage
cracking and a significant increase in the compressive strength” were reported with
increasing UFFA addition or decreasing w/ cm ratio.
Silica fume ( SF) is typically a byproduct of manufacturing silicon and ferrosilicon alloys,
i. e., a finely segregated residue captured from the oxidized vapor on top of the electric arc
14
furnaces. Most SF particles are amorphous and ultra- fine in size, averaging from 0.1 to
0.5 microns, or approximately one hundredth the size of the average cement particle.
Owing to its extreme fineness, large surface area and high silica content, SF is a
chemically reactive pozzolan and its use in cementitious systems has been specified by
ASTM C 1240 ( as shown in Appendix E3). Partial replacement of cement by SF up to
10% did not reduce the workability of fresh concrete, but slump loss with time was
observed to increase with replacement level at low w/ cm ratios [ 69]. As such, the SF
addition is often accompanied by the use of a superplasticizer, i. e., high- range water
reducer. Cong et al. [ 70] reported the partial replacement of cement by SF coupled with
the superplasticizer addition to increase the compressive strength of concrete, which was
largely attributed to the improved strength of its cement paste.
SF is known to considerably reduce the permeability of concrete by refining its
microstructure via both chemical and physical pathways, and thus greatly reduce the risk
of rebar corrosion in concrete. Selvaraj et al. [ 71] recently reviewed the influence of silica
fume on the factors relevant to the corrosion of reinforcement in concrete, including
chloride diffusion, carbonation, oxygen diffusion, pore solution pH, and electrical
resistivity of concrete. The partial replacement of cement by silica fume has been
reported to reduce the alkalinity of the pore solution and the chloride binding capacity of
hardened cement paste [ 72]. The reduction in pore solution pH is mainly due to the
pozzolanic reaction between silicon dioxide and the Portlandite [ 73]. The reduction in
chloride binding capacity by silica fume addition has been reported by other researchers
[ 51, 72, 74], as silica fume reduces the amount of aluminate phases in concrete that are
able to chemically bind chlorides and produces C- S- H that seem to have lower chloride
sorption than C- S- H generated from cement hydration [ 74]. These mechanisms can lead
to dramatic increase in the Cl-/ OH- ratio in the pore solution and may be responsible for
the reduction in the chloride threshold value of steel in concrete [ 21, 75]. Dotto et al. [ 76]
observed that the silica fume addition led to significant improvements in the corrosion
performance of the concrete as well as in the compressive strength of concrete, whereas
Page and Havdahl [ 77] observed slightly higher corrosion rates of steel in cement paste
containing silica fume.
Ground granulated blast- furnace slag ( GGBFS) is a byproduct of making iron and steel,
i. e., a fine powder grounded from the glassy, granular material that forms when molten
iron blast furnace slag is air quenched with water or steam. GGBFS features a fineness
similar to cement particles and contains very limited amount of crystals. GGBFS is
highly cementitious in nature and its use in mortar and concrete has been specified by
ASTM C 989. Partial replacement of cement by GGBFS up to 80% was observed to
reduce the compressive strength of concrete during the first 28 days while the later- age
strength increased with the slag replacement up to 60% [ 78]. Partial replacement of
cement by GGBFS up to 80% has demonstrated to improve the corrosion performance of
concrete and the 50% replacement level in concrete imparted the best corrosion
resistance, which featured the corrosion initiation time of steel rebar 3.2 to 3.8 times as
long as the control ( depending on the tricalcium aluminate [ C3A] content in cement) [ 79].
The effect of GGBFS addition on the sulfate resistance of concrete was more complex,
depending on the replacement level and the cement composition [ 79]. GGBFS was found
15
to considerably improve the pore structure of concrete, increase its chloride binding
capacity ( by forming more Friedel’s salt), and reduce its chloride diffusivity [ 80, 81].
While the slag addition improves both chemical and physical binding of chloride [ 51, 79,
83], it decreases the pH of the pore solution [ 81]. The effect of partial cement
replacement by slag on the chloride threshold value is still controversial, as Gouda and
Halaka [ 82] reported lower threshold values for slag concrete whereas Schiessl and Breit
[ 60] and Oh et al. [ 59] reported higher or similar threshold values respectively when
replacing cement with slag. Cheng et al. [ 81] investigated the corrosion behavior of
reinforced concrete prismatic beams subjected to sustained loadings ( 37% and 75% of the
ultimate load) and 3.5% NaCl solution and found the slag concrete to exhibit lower
corrosion rate for a given reduction percentage in flexural rigidity ( relative to the
control). The partial replacement of cement by GGBFS reduced the electrical charge
passing through the concrete ( during the rapid chloride penetration test) and the water
permeability of concrete. This was the result of GGBFS reacting with water and
Portlandite to form extra C- S- H gel and more refined microstructure. A study of slag
concrete after 25 years of exposure in a marine tidal zone [ 84] confirmed the beneficial
role of slag in dramatically reducing the chloride ion penetration, especially at relatively
high replacement levels ( 45% to 65%) and low w/ cm ratio ( 0.40).
Metakaolin ( MK) is a material obtained by calcining clay mineral kaolinite between 500-
800° C in an externally fired rotary kiln so that it loses water through dehydroxilization
( i. e., removal of chemically bonded hydroxyl ions). MK generally has particle size finer
than cement but not as fine as silica fume and features a two- dimensional order in crystal
structure ( as shown in Appendix E4). MK is a highly reactive aluminosilicate pozzolan
and its use in cementitious systems has been specified by ASTM C 618. The partial
replacement of cement by MK up to 20% was observed to greatly reduce the water
absorption of concrete by capillary action but slightly increase the water absorption of
concrete by total immersion [ 85]. The MK addition was found to increase early- age ( 1- 3
days) flexural strength of concrete by as much as 60%; and the finer MK ( with surface
area 25.4 vs. 11.1 m2/ g) showed higher reactivity and led to greater strength especially
for compressive strength of concrete with low w/ cm ratio ( 0.40). The MKs were found to
consume Portlandite via their pozzolanic reactivity and produce more refined pore
structure in concrete [ 86]. The sulfate resistance of both non- air- entrained and air-entrained
concrete was found to increase with the MK replacement level ( from 5% to
10% and 15%), by measuring expansion of concrete prisms and compressive strength
reduction of concrete tubes [ 87]. The replacement of cement or sand by MK ( 10% or
20% by weight of cement) can greatly reduce the chloride permeability, gas permeability
and sorptivity of concrete, by decreasing the mean pore size and improving uniformity of
the pore size distribution [ 88]. Both steady- and non- steady- state chloride diffusion tests
showed that the MK addition in Portland cement mortar tends to enhance the resistance to
chloride transport through both the hydrated cement matrix and the paste- aggregate
interfacial transition zone [ 89]. The MK addition was also found to compensate for the
low early- age compressive strength of concrete containing GGBFS [ 78].
16
1.2.3. Measuring the Chloride Ingress into Concrete
The chloride ingress into concrete and other cementitious materials is a complex
phenomenon involving multiple mechanisms. As such, a wide array of tests have been
developed and used to evaluate the resistance of chloride ion penetration into concrete. In
1997, Stanish et al. [ 90] conducted a literature review to synthesis the state of the art
pertinent to testing the chloride penetration resistance of concrete ( Table 1- 1). And since
then, there have been new advances in improving the test methods.
TABLE 1- 1 Summary of Chloride Penetration Test Methods [ 90]
There are two types of natural penetration experiments generally used to measure the
chloride diffusion coefficients of concrete. One is the steady- state diffusion tests, such as
the Diffusion Cell Test in which a concrete specimen is used to separate a chloride
solution from a chloride- free solution and periodical measurements of the chloride ion
content are conducted until a steady state condition is achieved. The other is from non-steady
state tests that involve the ponding or immersion of concrete specimen for a
specific duration before measuring the chloride penetration depth or profile, such as the
Salt Ponding Test [ 14]. The diffusion coefficients obtained are known as effective ( Deff)
and steady- state ( Ds), or apparent ( Dapp) and non- steady- state ( Dns) diffusion coefficient,
respectively [ 91, 92]. The ponding test has been standardized as AASHTO T 259 and
ASTM C 1543, involving the laborious analysis of chloride content at various depths of
17
the sample after 90 days of ponding, which apparently is not sufficient time for high-quality
concrete to produce meaningful chloride concentration profile.
Natural penetration tests ( based on ASTM standards) are very time- consuming,
especially for measuring the chloride diffusivity in high- quality concrete mixes. The
diffusion tests often take a minimum of 1 to 3 years of exposure in simulated weathering
conditions before any service life modeling can be conducted [ 93]. One way to accelerate
the ingress of chloride into concrete is to apply a pressure. There is little research on this
method, which exposes one face of the concrete to the chloride solution under pressure
and drives the chlorides into the concrete under both convection and diffusion [ 94].
Recently, there has been an increasing demand for rapid, reliable methods for testing the
chloride ion penetration resistance in a particular type of concrete and for testing the
corrosion risk of rebar in a particular environment.
In the last decades, electric field migration tests have become very popular as they can
greatly accelerate the chloride ingress into concrete. Rapid Migration Test ( RMT) is a
method to measure the electrical migration of chloride from one compartment with a
chloride solution to the other that is chloride- free [ 95]. The average depth of chloride
penetration is obtained by spraying a colorimetric indicator on the sample, and the value
is then divided by the product of the applied voltage and migration time to rate the
concrete permeability. Castellote et al. [ 92] developed a method to derive both Dns and Ds
from the migration test by monitoring the conductivity of the solution in the anodic
compartment ( destination solution that was initially distilled water). In our opinion,
however, this method may produce misleading results when used to test high- quality
concrete over a long time period, since the anolyte conductivity is very sensitive to
chemical changes induced by the electrochemical reactions at the anode and the leachates
from the test specimen.
Rapid Chloride Permeability Test ( RCPT) is a method that records the amount of charge
passed through a concrete sample in order to evaluate its permeability [ 96]. By
introducing the concept of ion mobility, the similarity between diffusion and migration
enables the determination of an ion diffusion coefficient from the migration tests. For
PCC and mortar with no or little minerals admixed, it has been shown that the total
charge passed is strongly correlated with the integral chloride content of ponding test [ 97]
and with the chloride diffusion coefficient obtained from an accelerated chloride
migration test [ 98, 99]. The RCPT has been standardized as AASHTO T 277 and ASTM
C 1202, involving the classification of chloride permeability of concrete based on the
charge in the first 6 hours, which again is not sufficient time to differentiate high- quality
concrete mixes. Furthermore, the electrical charge passed in the test is related to all ions
in the pore solution, not just chloride ions [ 90]. In addition, RCPT is not suitable for
evaluating the chloride permeability of concrete with supplementary cementing materials
[ 15, 100, 101], since the results may be significantly biased due to the change in the
chemical composition of the pore solution [ 91, 102].
Accelerated Chloride Migration Test ( ACMT) can be considered a modified version of
RMT and RCPT, which periodically measures the accumulative chloride ion
18
concentration in the destination compartment either by the potentiometric titration
method [ 103] or using a chloride sensor [ 98, 99]. The test lasts until significant chloride
ion concentration is detected in the destination compartment, which could be hours, days,
or weeks depending on the thickness and quality of the test specimen and the applied
voltage. Cho and Chiang [ 103] investigated the chloride diffusivity in concrete specimens
with various w/ c ratio ( 0.35 to 0.65) and slag content ( 0% to 70%). They found good and
very poor correlation between the charge passed and the non- steady- state diffusion
coefficient ( Dns) obtained from the ponding test, for concrete with and without slag
respectively. For both types of concrete, there was a linear correlation between the
steady- state diffusion coefficient ( Ds) obtained from the ACMT and the Dns obtained
from the ponding test, suggesting the ACMT to be a reliable accelerated test method. If
the applied voltage is too high ( e. g., 60 V), the Joule effect may lead to a higher value of
electrical charge passed during RCPT or a higher Ds value during ACMT, i. e., the
temperature increase of the test solutions [ 96], which can be mitigated by significantly
increasing the volume of test solutions ( e. g., from 250 mL to 4.5 L) [ 103]. Furthermore,
the geometric shape of the test cell and the resistivity of the concrete specimen could
affect the test results [ 104, 105]. Vivas et al. [ 106] investigated the chloride diffusivity in
19 concrete mixes prepared with materials typically used in construction in the state of
Florida. They found that the RMT test ( NordTest NTBuild 492) results had similar or
better correlation with the 364- day bulk diffusion test ( NordTest NTBuild 443) than those
from the RCPT ( ASTM C 1202) or the surface resistivity test ( FM 5- 578) and were less
affected by the presence of SCMs in concrete.
1.3. Challenges in Assessing Concrete Durability from Its Chloride Diffusivity
The length of the corrosion propagation stage in concrete is usually found to be relatively
short, typically a few years. As a result, much of the emphasis on achieving concrete
durability of 75 years or longer is put on achieving a long corrosion initiation stage [ 107],
which is a function of the chloride transport properties of concrete ( usually the diffusion
coefficient), the surface chloride content dictated by the environment, the concrete cover
thickness, and the chloride threshold concentration determining the onset of active
corrosion. Note that Pettersson [ 108] argued that the propagation period could be as long
as 50 years or more for high performance concrete featuring high electric resistivity and
very limited oxygen availability.
Both the chloride ingress into concrete and the subsequent corrosion initiation of rebar in
concrete are complex processes, which are influenced by numerous factors. Therefore,
challenges are inherent in assessing concrete durability from its chloride diffusivity,
mainly pertinent to the determination of chloride threshold and the quantification of
chloride binding effect.
1.3.1. Chloride Threshold
Chloride threshold of rebar in concrete, Clth, can be defined as the content of chloride at
the depth of the rebar that is necessary to sustain localized breakdown of its passive film
and hence initiate its active corrosion [ 109]. The time it takes for chloride ions from
19
external sources ( marine environments or deicing salt applications) to reach Clth at the
rebar depth is defined as time- to- corrosion, Ti. The Clth is an important parameter in
modeling and predicting the Ti and subsequently in assessing the service life of reinforced
concrete in chloride- laden environments [ 81, 110].
The Clth data in published literature scatter over a wide range of values [ 111- 113]. One
possible reason is that the chloride threshold has different definitions and measurement
methods [ 114- 116]. The chloride- to- hydroxyl ionic concentration ratio ([ Cl-]/[ OH-]) has
been traditionally considered to be a more reliable indicator than the chloride
concentration ( often expressed as total chloride content by the weight of cement or
concrete or free chloride concentration in concrete pore solution), considering that the
competition between aggressive Cl- and inhibitive OH- governs the pitting and
repassivation of steel. Research in aqueous solutions has indicated that for chloride-contaminated
concrete the pitting corrosion occurs only above a critical [ Cl-]/[ OH-] ratio
[ 115]. Through a probability simulation model, the threshold [ Cl-]/[ OH-] for corrosion of
bare steel rods in high pH solutions was once predicted to be 0.66 in the presence of
oxygen bubbles attached to the steel and 1.4 in the case of air. Such result agreed
favorably with experimental data. In the same model, it was concluded that the threshold
ratio should be about 1.4 for typical reinforced concrete and in excess of 3 for high-quality
concrete with minimal air voids [ 10]. A number of studies [ 61, 117- 120] exposed
reinforcing steel bars to simulated concrete pore solutions and revealed that the threshold
[ Cl-]/[ OH-] ratio increased with increasing pH. The threshold Cl/ OH ratio in mortars has
reported higher results ( 1.17- 3.98) than that found in synthetic pore solution ( 0.25- 0.8)
[ 112]. Recently, Ann and Song [ 115] argued that the ratio of total chloride content to acid
neutralization capacity, [ Cl-]/[ H+], best presents the chloride threshold level since it takes
into account “ all potentially important inhibitive ( cement hydration products) and
aggressive ( total chloride) factors”. The different methods used to assess the chloride
content or its profile in concrete contributed to the variability in reported Clth values.
Traditionally, the coring method is commonly used, which involves acquisition of one or
more cores from sound concrete between reinforcements at the time of active corrosion
initiation. The cores are sliced and analyzed for their chloride content, and the chloride
content in the slice near the rebar depth is defined as Clth. Recently, both experimental
[ 131] and modeling [ 125, 130, 132, 133] studies unraveled that chloride content at the top
of the rebar trace was higher than that at the same depth away from the rebar, owing to
the relatively low content of coarse aggregates in the vicinity of the rebar [ 128] and the
rebar serving as a physical barrier to chloride migration. Thus, it is more reasonable to
express Clth with the chloride content on the top of rebar trace than that acquired from the
core sample.
The lack of universally accepted chloride threshold value is also attributable to the
numerous factors that affect steel corrosion in concrete, such as: the pH of concrete pore
solution, the electrochemical potential of the steel, and the physical condition of the
steel/ concrete interface. The pH of concrete pore solution mainly depends on the type of
cement and additions and the carbonation level of concrete [ 121- 124]. The potential of
the steel is not only related to the steel type and surface condition ( e. g., roughness) but
also the oxygen availability at the steel surface; the latter is affected by the moisture
20
content of concrete [ 61, 119, 125]. The physical condition of the steel/ concrete interface
( especially entrapped air void content) was found to be more dominant in the Clth than
chloride binding or buffering capacity of cement matrix or binders [ 115]. Voids that can
be normally found in real structures due to incomplete compaction may weaken the layer
of cement hydration products deposited at the steel/ concrete interface and thus may favor
local acidification required for sustained propagation of pits. The presence of air voids, as
well as crevices and microcracks, may decrease the chloride threshold [ 126- 129]. In
addition, the presence of sulfate ions, the temperature and the concrete mix proportions
and quality may affect the chloride threshold [ 113, 114, 120, 129]. Li and Sagüés [ 120]
listed a wide array of internal and external factors defining the Clth, such as: the
composition, surface condition, and configuration of rebar; the concrete chemistry ( type
and amount of cement and admixtures, type and porosity of aggregates, w/ c ratio, degree
of hydration, etc.); the type and source of chloride; and the service environment
( humidity, temperature, cracking of concrete, etc.). Angst et al. [ 114] summarized the
state of the art in the Clth research in a recent review.
Furthermore, it has been argued that the Clth ( and Ti) should be treated as a distributed
parameter represented by a probability function, in light of the statistical nature of the
processes involved ( e. g., chloride ingress and pitting initiation) and the inherent
heterogeneities of the concrete matrix [ 120, 128, 129]. Li and Sagüés [ 120] suggested the
incorporation of a Clth variability term in the service life prediction procedures and
durability models. Hartt and Nam [ 121] reported a range of values for Clth and Ti with
seemingly identical slabs and the same exposure condition, as variability was introduced
by micro- structural factors such as the size and distribution of entrapped air voids, and
the arrangement of aggregates which can significantly affect the tortuosity of chloride
ingress path. Currently, there is limited research on the probabilistic nature of how mix
design and other factors impact corrosion initiation ( as indicated by Clth and Ti) [ 121, 125,
130].
1.3.2. Chloride Binding
In concrete, chlorides can exist either in the pore solution, chemically bound with
concrete C3A ( tricalcium aluminate) or C4AF phases ( e. g., Friedel’s salt,
3CaO  Al2O3  CaCl2  10H2O), or physically held to the surface of hydration products
( e. g., adsorption on C- S- H) [ 134- 136]. Chloride binding removes chloride ions from the
pore solution, and slows down the rate of penetration [ 137]. With water- soluble and
acid- soluble chlorides referred to as free and total chlorides respectively, the total
chloride diffusivity was found to be near three times the free chloride diffusivity [ 19].
In a Florida DOT study, the relationship between bound and free chlorides was found to
follow the Langmuir adsorption isotherm [ 12]. While previous studies [ 36, 138] suggest
that only free chloride ions in the pore solution are responsible for initiating corrosion
of the steel, Glass et al. [ 111, 139, 140] indicated that bound chlorides may also present
a significant risk to steel. One possible reason is that a large part of bound chlorides are
released as soon as the pH drops to values below 12 [ 5].
21
Chloride binding further complicates the determination of the threshold [ Cl-]/[ OH-] to
initiate corrosion of steel in concrete [ 141], and the chloride binding and pH of pore
solution are two inter- related parameters. It has been observed that the pH of NaCl-containing
alkaline solution increases as the chloride binding increases [ 135]. Reducing
the pH in concrete may destabilize the chloroaluminate and thus reduce the percent of
bound chloride [ 5, 115], and carbonation of concrete can reduce the chloride binding
capacity [ 116, 142] and facilitate the chloride intrusion [ 143]. Chloride binding
evidently decreases with increasing OH- above pH 12.6 and a decrease in pH can thus
result in decreasing [ Cl-]/[ OH-] [ 144].
The chloride binding capacity is affected by numerous factors, such as the C3A and
alkali contents of cement [ 145], use of mineral admixtures [ 19, 49- 56, 71, 73, 78, 79],
cation of the chloride salt [ 146], temperature, and degree of hydration [ 137].
1.4. Chloride Transport in Concrete and Service Life of Reinforced Concrete – A
Modeling Perspective
This work started with a comprehensive literature review on topics relevant to this study.
As detailed in Appendix A, we synthesized the information on existing research related to
the computational models to simulate the transport of species in aqueous solution, water-unsaturated
and water- saturated cementitious materials, and the service life modeling of
reinforced concrete in chloride- laden environments.
1.5. A Phenomenological Model for the Chloride Threshold of Pitting Corrosion of
Steel in Simulated Concrete Pore Solutions
As detailed in Appendix B, we also conducted a systematic study aimed to provide
quantitative understanding of the fundamental factors that influence the chloride
threshold of pitting corrosion of steel in concrete, by conducting a set of laboratory tests
to assess the corrosion potential ( Ecorr) and pitting potential ( Epit) of steel coupons in
simulated concrete pore solutions. With the aid of artificial neural network ( ANN), the
laboratory data were then used to establish a phenomenological model correlating the
influential factors ( total chloride concentration, chloride binding, solution pH, and
dissolved oxygen concentration) with the pitting risk ( characterized by Ecorr - Epit). Three-dimensional
response surfaces were then constructed to illustrate such predicted
correlations and to shed light on the complex interactions between various influential
factors. The results indicate that the threshold [ Cl-]/[ OH-] of steel rebar in simulated
concrete pore solutions is a function of dissolved oxygen concentration, pH and chloride
binding, instead of a unique value. The limitations and practical implications of the
research findings were also discussed.
1.6. Modeling Cathodic Prevention for Unconventional Concrete in Salt- laden
Environment
As detailed in Appendix C, we conducted numerical studies to provide a modeling
perspective relevant to the use of cathodic protection ( CPre) for SCMs- containing
22
concrete in salt- laden environment. Based on the experimentally obtained concrete
resistivity and chloride diffusion coefficient data, the Nernst- Planck equations were used
to investigate the influence of applied voltage ( magnitude, direction, and interruption),
surface chloride concentration, and concrete mix design on the effectiveness of cathodic
prevention and the distribution of ionic species in protected concrete. The modeling
results revealed that the direction of applied electric voltage has significant effect on the
distributions of electrical potential and hydroxyl ions in the reinforced concrete,
confirming the benefits of cathodic prevention in significantly increasing hydroxyl
concentration near rebar and in slowing down the ingress of chloride ingress into
concrete. The performance of intermittent CPre was found to be constrained by the
variations in concrete resistance from the anode to the cathode. The model was also
useful in illustrating the temporal and spatial evolutions on rebar surface in terms of
oxygen, hydroxyl and chloride concentrations on and electrical potential of top rebar as
well as such evolutions in concrete domain in terms of concrete resistivity and current
density for each mix design. The results shed light on the fundamental processes defining
the performance of CPre for new unconventional concrete in salt- laden environment.
1.7. Study Objectives
The objectives of this research are to validate chloride diffusion coefficients of mineral
admixture concrete mix designs currently developed by the Caltrans for corrosion
mitigation, and to verify the adequacy of existing measures to mitigate corrosion caused
by exposure to marine environments and deicing salt applications. To this end, this
research includes a comprehensive literature review on relevant topics, a laboratory
investigation and a modeling effort. The laboratory investigation examines the
compressive strength of and chloride diffusivity in mortar and concrete samples with
cement partially replaced by various minerals ( class F and class N FA, UFFA, SF, MK
and GGBFS), the porosity of mineral concretes, the freeze- thaw resistance of mineral
mortars in the presence of deicers, the effect of SCMs on the chloride binding and
chemistry of the pore solution in mortar, as well as the pitting risk of steel rebar in
simulated pore solutions with various chloride concentration, chloride binding, pH and
dissolved oxygen concentration. The modeling effort explores the important features of
ionic transport in concrete and develops a two- dimensional finite- element- method ( FEM)
model coupled with the stochastic technique. The numerical model is then used to
examine the service life of reinforced concrete as a function of mix design ( i. e., partial
replacement of cement by mineral admixtures), concrete cover depth, surface chloride
concentrations, and presence of cracks and coarse aggregates.
1.8. How This Report Is Organized
The following chapter will present the methodology, results and discussion pertinent to
the laboratory investigation of mortar and concrete samples. Chapter 3 presents the
methodology, results and discussion pertinent to the modeling effort. Finally, Chapter 4
summarizes the key findings from both the laboratory and modeling components of this
work, followed by suggestions and recommendations for implementation by the Caltrans.
Appendices conclude this report.
23
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32
CHAPTER 2. LABORATORY INVESTIGATION
This chapter presents the methodology, results and discussion pertinent to the laboratory
investigation of mortar and concrete samples. Various laboratory tests were conducted to
investigate the compressive strength of and chloride diffusivity in mortar and concrete
samples with cement partially replaced by various minerals ( class F and class N FA,
UFFA, SF, MK and GGBFS), the porosity of mineral concretes, the freeze- thaw
resistance of mineral mortars in the presence of deicers, and the effect of SCMs on the
chloride binding and chemistry of the pore solution in mortar.
2.1. Experimental
2.1.1. Sample Preparation
In light of the representative concrete mixes and chloride exposure conditions in
California, a preliminary design for the laboratory investigation was developed, in the
form of a matrix of 18 concrete mix designs that need to be evaluated ( see Table 2- 2). All
these concrete mix designs feature a w/ cm ratio of 0.40. The concrete mix design without
any mineral admixtures is used as a control. These mix designs were determined in close
consultation with the Caltrans Corrosion Technology Branch staff. On the basis of Table
2- 2, multiple trials were conducted in order to achieve reasonable workability of fresh
concrete ( slump) for each mix design. For this study, we used an ASTM specification
C150- 07 Type I/ II low- alkali Portland cement from the Ash Grove Montana City Plant
( Clancy, MT) and its chemical composition and physical properties are provided in
Appendix E7. The properties of the mineral admixtures used are provided in Appendices
E1- E6. Coarse aggregates ( with maximum size of 3/ 4  or 19 mm) and fine aggregates
( clean, natural silica sand) were purchased from the JTLGroup ( Belgrade, MT). Glenium
3030TM and Micro- AirTM were used as the ASTM C 494 Type A/ F water reducing agent
and the ASTM C 260 air- entraining agent respectively and at the dosage per the
instructions.
After the trials, the two Class N fly ash designs ( at 25% replacement level) were excluded
from further investigation with approval of the Caltrans technical panel, since these two
mixes could not achieve desired slump and air content with the specified w/ cm ratio of
0.4 even with the excessive amounts of multiple water- reducers. This left 16 concrete
mixes for the study as shown in Table 2- 3. These concrete mixes had a coarse- aggregate-to-
cementitious- materials ratio varying between 2.17 and 2.86 and a coarse- to- fine-aggregates
ratio between 1.51 and 1.54. Such variations were necessary in order to
achieve reasonable slump and air content, similar to the field construction scenarios
during batching operations. Note that the actual air content achieved deviated from the
target air content in Table 2- 2 in spite of the multiple trials for each mix design. We also
noticed that concrete made using a smaller lab mixer with same formulation usually had
lower air content than using a larger lab mixer.
For each mix design, at least three replicate 12   by 6   ( diameter 305 mm  height 152
mm) concrete cylinders and at least three replicate 4  by 8  ( diameter 102 mm  height
33
203 mm) compression cylinders were prepared. The coarse aggregates and fine
aggregates were oven- dried and then we added the amount of water twice as much as
their absorption capacity ( e. g., 1.8%). The aggregates were then soaked for 24 hours to
ensure that they had fully absorbed moisture and had moisture in excess of the surface-saturated-
dry ( SSD) condition. The saturated aggregates and the excessive water were
used in the mix, taking into account the excessive water when calculating the w/ c ratio.
The fine and coarse aggregates were added to the 2- cubic- feet ( 57- L) mixer and mixed
until a homogeneous mixture was obtained. Then the cement was added and mixed again
until a homogeneous mixture was obtained. Next, water was added from a graduated
cylinder and mixed until the concrete is homogeneous and of the desired consistency. The
batch was remixed periodically during the casting of the test specimens and the mix
container was covered to prevent evaporation. Slump and air content measurements were
performed by the ASTM C 143 and C 173 methods respectively, to check the workability
and quality of the freshly mixed concrete; and the data are shown in Table 2- 3. Fresh
concrete was cast into hollow poly( vinyl chloride) piping cylinders and then carefully
compacted to minimize the amount of entrapped air. The cylindrical samples were
demolded after curing for 24 hours with over 90% relative humidity. After demolding,
the samples were cured in the moist cure room ( with over 90% relative humidity) for
another 359 days before the accelerated chloride migration test. For testing of chloride
diffusivity, slice specimens with diameter of 2  ( 51 mm) and thickness of 1  ( 25 mm)
were cored from the center of cured cylinders to minimize possible effects of surface
evaporation and air entrapment on the permeability of slice specimen. Cores were
removed from the concrete according to the ASTM C42/ C 42 M ( 2004) Standard Test
Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. The
specimen thickness was chosen based on two considerations. It is thick enough to
reasonably represent the heterogeneity nature of the concrete and to consider the
maximum aggregate size ( 3/ 4  ). It is no too thick so that the accelerated chloride
migration test can be completed in reasonable time frame.
We also prepared 9 mortar mixes ( mixes 1, 3, 5, 7, 9, 11, 13, 15, and 17 in Table 2- 3
without any coarse aggregates, water- reducer, or air- entraining agent. The w/ cm ratio of
the mortar samples was set at 0.45 instead of 0.40, in light of workability concerns. For
each mix design, at least three replicate 2   by 4   ( diameter 51 mm  length 102 mm)
cylinders for diffusivity testing, at least sixteen replicate 1⅞  by 1 ½   ( height 48 mm 
diameter 38 mm) cylinders for freeze- thaw testing, and at least nine replicate 2  by 4 
( diameter 51 mm  length 102 mm) compression cylinders were prepared. This aims to
shed light on the role of coarse aggregates and to better interpret the chloride diffusion
data in concrete containing various types and amounts of mineral admixtures. For mortar
samples, cement is mixed with water at a low speed hand mixer for 5 minutes.
Subsequently, fine aggregates, with a maximum size of 1.18 mm in diameter, were
added, after which the slurries were stirred for 3 minutes. The fine aggregates were
prepared to SSD condition in advance. All the slurries were cast into hollow poly( vinyl
chloride) piping cylinders and then carefully compacted to minimize the amount of
entrapped air. The cylindrical samples were demolded after curing for 24 hours with over
90% relative humidity. After demolding, the samples were cured with over 90% relative
humidity for another 89 days before the accelerated chloride migration test. For testing of
34
chloride diffusivity, slice specimens with a thickness of 8 mm were cut from the center of
cured cylinders to minimize possible effects of surface evaporation and air entrapment on
the permeability of slice specimen. This was done using a low- speed saw equipped with a
diamond blade.
TABLE 2- 2 Preliminary design of experiments to study the influence of concrete mix
design parameters on the chloride penetration resistance and durability of concrete,
including type and amount of mineral replacement and entrained air content.
35
TABLE 2- 3 Mix design parameters and the properties of concrete samples containing
various types and amounts of mineral admixtures.
1 2 3 4 7 8 9 10
Portland Cement ( lb) 75 75 56 56 56 56 56 56
Fly Ash ( Class F) ( lb) 19 19 15 15 15 15
Silica Fume ( lb) 4 4
Metakaolin ( lb) 4 4
Ultra Fine Fly Ash ( lb)
Blast Furnace Slag ( lb)
Micro Air ( ml) 22 33 33 33
Glenium 3030 ( ml) 150 165 33 24 133 44 200 133
Water ( lb) 28 25 28 28 24 26 25 24
Fine Aggregate ( lb) 106.05 129.5 139.3 127.3 139.13 127.22 139.41 127.5
Moisture Content (%) 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6
Course Aggregate ( lb) 162.4 199.3 210.3 192.8 214.5 194 213.5 196.4
Moisture Content (%) 0.55 2.106 0.667 0.96 2.769 1.66 2.089 2.708
Slump ( in) 2.5 3 2.5 3.5 3 4 4.5 3.5
Air Content (%) 2.5 6.5 2.5 5.5 3.75 5.6 2.75 6.5
Volume ( ft3) 3 3 3 3 3 3 3 3
Strength @ 90 days ( psi) 10367 7834 9404 5654 10020 5841 9557 6833
Standard Deviation ( psi) 152 283 201 28 233 261 251 337
Ingredients Mix Number
11 12 13 14 15 16 17 18
Portland Cement ( lb) 68 68 68 68 68 68 38 38
Fly Ash ( Class F) ( lb)
Silica Fume ( lb) 8 8
Metakaolin ( lb) 8 8
Ultra Fine Fly Ash ( lb) 8 8
Blast Furnace Slag ( lb) 38 38
Micro Air ( ml) 22 22 21 22
Glenium 3030 ( ml) 266 238 255 238 150 150 155 166
Water ( lb) 28 28 28 28 25 25 27 27
Fine Aggregate ( lb) 140.3